According to the current Encyclopedia Britannica (1974 Edition), under the entry "Sails and Sailing Ships",
In 1924 a wholly new kind of sailing vessel, the rotor ship, was extensively tested. Its German inventor, Anton Flettner, based his idea on the discovery that the wind pressure on a revolving cylinder was considerably greater than on a conventional sail. Flettner began by installing in a small freighter two cylinders 50 feet (15 meters) high, which were spun by an electric motor. Steering by controlling the spinning of the rotors, he reached a speed of nine and a half knots. He made further tests with three cylinders 60 feet (18 meters) high in a larger ship but found he could obtain satisfatory results only when all conditions were favourable. The idea was not pursued, and there seems to have been no other invention using wind power in such a radical way.
In 1926, Flettner published his work in book form, and Albert Einstein, in his Essays in Science, devotes a full essay to "The Flettner Ship".
The basic Flettner sail invention is described in U.S. Pat. No. Re. 18,122, reissued July 7, 1931. Several U.S. patents issued before and since show windmills associated with vehicles, vehicles with rotatable or movable surfaces for achieving some surface effect benefit, and applications for the Flettner invention in sailing and other fields.